nuther question...
As I look at my list of some two dozen frames (don't ask why I have two dozen+ frames), I notice that ST to TT ratio seems to increase as the ST size increases.

So, a 50cm ST frame typically has a TT greater than its ST (TT = 52, even 54cm),

then when I get to the middle of the pack (ST=56 or 57) the TT matches (I think this is called "Square"? as in "a 56square is a 56STx56TT) then in the bigger frame sizes the TT drops down smaller than the ST (so a 62cm ST frame has a 59 or 60cm TT)

This can't be based on body dimensions.
Is there some constraint in framebuilding that makes this change in ST:TT ration change so?

actually, it is based on body dimensions for the most part. If you take a large number of humans and sit them at a table, you will see much less variation in height than you do in standing height.

I'm not sure I buy that. Obviously seated people are half the height, so half the variance, but that assumes the upper body range difference is the same as the bottom.

I had three reasons, but they were just guesses, I'm not convinced. One is based on the idea there is a cycling type who in performance male version has long legs and smaller upper body, lots of power to push, but no more upper body weight than necesarry. Two, the idea is that the tubes are fixed length so they run out of materials in the longer sizes and there are constraints, but since tubes are available in a variety sizes, seems unlikely. 3, is the idea that there are certain markers that buyers are looking for like a wheel base length or frame weight, and the longer sizes get squeezed. If a big guy can get onto the shorter top tube length, maybe he wants the more nimble wheel base.

There's a good explanation of this phenomenon in the C.O.N.I. Manual, Part II, Chapter 5, "Modalities for constructing a frame to measure."

https://www.sandcreeksports.com/docum...ramesizing.pdf

Note that in Table 1 on page 129, the seat tube length for lower limb (a) of 89mm should be 56.7mm, not 65.7mm (the 5 and 6 are transposed).

The whole manual can be downloaded from HERE.

I love that book, I need to go back and read it again. I was once accused of sleeping with it under my pillow